Curing a lost circulation zone in a wellbore

ABSTRACT

An example method includes pumping fluid into a wellbore to fill, at least partly, a region in a lost circulation zone, with the fluid having a first density; and after pumping the fluid, pumping cement slurry into the wellbore. The cement slurry has a second density. The first density is greater than, or equal to, the second density, which causes the fluid to prevent, at least partly, the cement slurry from mixing with other fluid in the lost circulation zone for at least a period of time.

TECHNICAL FIELD

This specification relates generally to example processes for curing alost circulation zone in a wellbore.

BACKGROUND

In a well, such as an oil well, a lost circulation zone is a region in asubterranean formation that inhibits, or prevents, return of mud orother materials following introduction of drilling fluid. For example,during drilling and completion of a well, drilling fluid is introducedinto the wellbore. Then, mud and other materials from the wellbore flowback to the surface of the well. However, in a lost circulation zone,the introduction of drilling fluid into the wellbore does not produce acorresponding flow back to the surface of the well.

There can be various causes for lost circulation zones. In some cases,the formation may have high permeability, may have a high porosity andmay have a less-than-normal hydrostatic pressure. In some cases, theformation may contain faults, such as fractures, into which the drillingfluid escapes, thereby interrupting the circulation of fluids out of awellbore. Such lost circulation zones can adversely affect forwarddrilling operations.

In some cases, in order to continue drilling the well to a target depthor to a next casing point, these lost circulation zones should beremedied. In some cases, lost circulation material (LCM) pills, cementplugs, and X-linked polymer plugs have be used in attempts to remedylost circulation zones.

SUMMARY

An example method for at least partially curing a lost circulation zoneincludes pumping fluid into a wellbore to fill, at least partly, aregion in a lost circulation zone, where the fluid has a first density;and after pumping the fluid, pumping cement slurry into the wellbore.The cement slurry has a second density. The first density is greaterthan, or equal to, the second density, which causes the fluid toprevent, at least partly, the cement slurry from mixing with other fluidin the lost circulation zone for at least a period of time to allow theslurry to harden. The example method may include one or more of thefollowing features, either alone or in combination.

The fluid may be a first fluid. The example method may include formingthe wellbore by drilling through a formation using a second fluid. Thesecond fluid may have a third density, and the first density may begreater than the third density. Pumping the first fluid may cause thefirst fluid to displace at least part of the second fluid in thewellbore.

The lost circulation zone may include at least part of a formationcontaining formation fluid, which may constitute the other fluid. Thefirst fluid may enter the formation and may force at least part of theformation fluid away from a region adjacent to the wellbore. The firstfluid may isolate, at least partly, the formation fluid from the cementslurry. The fluid pumped may have a first viscosity and the formationfluid may have a second viscosity. The first viscosity may be greaterthan the second viscosity.

The fluid pumped-in may prevent the cement slurry from mixing with theother fluid in the lost circulation zone for sufficient time to allowthe cement slurry to harden into cement. Pumping the cement slurry maybe performed immediately following pumping the fluid. The period of timemay be an amount of time that it takes for the cement slurry to set to athreshold amount.

An example method is used to form a well through a subterraneanformation having a fracture containing a formation fluid. The examplemethod includes pumping a first fluid into a wellbore of the well toforce at least some of the formation fluid away from a region of thesubterranean formation that is adjacent to the wellbore, with the firstfluid having a first density. The example method also includes pumpingcement slurry into the wellbore so that the cement slurry is at leastadjacent to the first fluid. The cement slurry has a second density. Thefirst density may be greater than the second density, which causes thefirst fluid to act as a barrier, at least partly, between the formationfluid and the cement slurry for at least a period of time that is basedon a setting time for the cement slurry. The example method may includeone or more of the following features, either alone or in combination.

The first fluid may have a first viscosity and the formation fluid mayhave a second viscosity. The first viscosity may be greater than, orequal to, the second viscosity. The period of time may be long enough toenable the cement slurry to set to a threshold amount. The first fluidmay be pumped into the wellbore by a pumping mechanism. The pumpingmechanism may operate at a maximum pump speed to pump the fluid. Pumpingthe cement slurry may be performed immediately following pumping thefirst fluid. Pumping the first fluid and pumping the cement slurry maybe part of a single continuous operation.

A volume of the cement slurry that is pumped may be based on an amountof the cement slurry needed to fill a lost circulation zone thatincludes the region of the subterranean formation. The fracture may bepart of a lost circulation zone in the wellbore. The formation fluid mayhave a third density. The second density may be greater than the thirddensity.

An example system includes a detector to identify a lost circulationzone in a wellbore of a well; and one or more pumps that arecontrollable to perform operations that include: pumping first fluidinto the wellbore to fill at least part of the lost circulation zone,with the first fluid having a first density; and pumping cement slurryinto the wellbore. The cement slurry has a second density. The firstdensity is greater than the second density causing the first fluid atleast partly to prevent the cement slurry from mixing with other fluidin the lost circulation zone for at least a period of time. The examplesystem may include one or more of the following features, either aloneor in combination.

The example system may include a casing inside at least part of thewellbore. The wellbore may extend through a subterranean formationhaving a feature that causes the lost circulation zone. The lostcirculation zone may contain formation fluid, and the other fluid mayconstitute the formation fluid. The example system may include a cementretainer connected to the casing to direct the cement slurry into thewellbore. The one or more pumps may be controllable to pump the firstfluid into the wellbore using a force that is sufficient to cause thefirst fluid to enter the feature and to displace at least some of theformation fluid at a region of the feature that is adjacent to thewellbore. The one or more pumps may be controllable to pump the cementslurry using a force that is sufficient to cause at least some of thecement slurry to enter the feature between the first fluid and wellbore.

The detector may include a computing system programmed to receiveinformation representing flow of fluid out of the wellbore followingflow of fluid into the wellbore. The period of time may be an amount oftime that it takes for the cement slurry to set to a threshold amount.The period of time may be at least two hours, at least three hours, atleast four hours, or at least five hours. The first fluid may have afirst viscosity. The other fluid may have a second viscosity. The firstviscosity may be greater than the second viscosity.

Any two or more of the features described in this specification,including in this summary section, may be combined to formimplementations not specifically described in this specification.

All or part of the processes, methods, systems, and techniques describedin this specification may be controlled by executing, on one or moreprocessing devices, instructions that are stored on one or morenon-transitory machine-readable storage media. Examples ofnon-transitory machine-readable storage media include, but are notlimited to, read-only memory, an optical disk drive, memory disk drive,random access memory, and the like. All or part of the processes,methods, systems, and techniques described in this specification may becontrolled using a computing system comprised of one or more processingdevices and memory storing instructions that are executable by the oneor more processing devices to perform various control operations.

The details of one or more implementations are set forth in theaccompanying drawings and the description subsequently. Other featuresand advantages will be apparent from the description and drawings, andfrom the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of an example wellbore having normalcirculation.

FIG. 2 is a cross-section of an example wellbore that has lostcirculation.

FIG. 3 is a cross-section of an example wellbore containing cementslurry in a lost circulation zone of the wellbore.

FIG. 4 is a cross-section of an example wellbore containing ahigh-density fluid between formation fluid and cement slurry in a lostcirculation zone of the wellbore.

FIG. 5 is a cross-section of an example wellbore illustrating a firststage of curing a lost circulation zone by pumping high-density fluidinto the wellbore.

FIG. 6 is a cross-section of an example wellbore illustrating a secondstage of curing a lost circulation zone by pumping cement slurry intothe wellbore.

FIG. 7 is a flowchart showing an example process for curing a lostcirculation zone in a wellbore.

Like reference numerals in different figures indicate like elements.

DETAILED DESCRIPTION

Described in this specification are example processes for curing a lostcirculation zone in a wellbore using high-density fluid and cementslurry. In some implementations, the high-density fluid includes fluidcomprised of water- or oil-based mud; however, any appropriate fluidsmay be used. In some implementations, high-density fluid includes anyfluid having a density that is greater than, or equal to, a density ofcement slurry that is used in cementing operations. As describedsubsequently, the high-density fluid is pumped into a lost circulationzone of the wellbore, and enters into subterranean formations in thelost circulation zone. The high-density fluid displaces fluid in theformation, referred to as formation fluid, by forcing the formationfluid from a region proximate to the wellbore further into theformation. Cement slurry is then pumped into the lost circulation zoneof the wellbore. The cement slurry has a density that is less than, orequal to, a density of the high-density fluid, and fills a space in thewellbore adjacent to the high-density fluid. Because the high-densityfluid has a density that is greater than, or equal to, a density of thecement slurry, the buoyancy effect causes the high-density fluid toinhibit migration, dilution, and contamination of the cement slurry.

Migration, dilution, and contamination may continue at a contact areabetween the high-density fluid and formation fluid inside of the lostcirculation zone, away from the cement slurry. The high-density fluid,however, acts as a barrier or buffer between the formation fluid and thecement slurry. Interaction between the high density fluid and theformation fluid occurs through substitution of one liquid for the other,providing appropriate time to allow the cement slurry to set, or toharden. In some implementations, after the cement slurry hardens, thewellbore is effectively isolated from the lost circulation zone,allowing to continue the drilling operation.

The time needed for the cement slurry to achieve a predefined hardnessmay vary based on a number of conditions including, but not limited to,the composition of the cement slurry, the temperature in the wellbore,and the pressure in the wellbore. In some examples, it may take two totwelve hours for cement slurry to set; however, that number may bedifferent for different conditions. In some implementations, setting ofthe cement slurry may include hardening to less than complete hardnessof cement. In some implementations, setting of the cement slurry mayinclude hardening to complete hardness of cement.

Referring to FIG. 1, to produce a well, a drill 10 bores through earth,rock, and other materials to form a wellbore 12. A casing 14 supportsthe sides of the wellbore. The drilling process includes, among otherthings, pumping drilling fluid 16 down into the wellbore, and receivingreturn fluid 18 containing materials from the wellbore at surface 20. Insome implementations, the drilling fluid includes water- or oil-basedmud and, in some implementations, the return fluid contains mud, rock,and other materials to be evacuated from the wellbore. This circulationof fluid into, and out of, the wellbore, may occur throughout thedrilling process. In some cases, this circulation is interrupted, whichcan have an adverse impact on drilling operations. For example, loss ofcirculation can result in dry drilling, which can damage the open hole,and result in possible loss of the open hole and the drilling bottomhole assembly, the drill string, or the drilling rig itself. In somecases, loss of circulation can cause a blow-out and result in loss oflife.

There are degrees of lost circulation that may result. For example, atotal loss of circulation occurs when no return fluid reaches thesurface following introduction of drilling fluid into the wellbore. Apartial loss of circulation occurs when than a predefined minimum amountof return fluid reaches the surface following introduction of drillingfluid into the wellbore. In some implementations, the techniquesdescribed in this specification may be used to cure a total loss ofcirculation. However, the techniques are not limited only to curingtotal losses of circulation, and may also be used to cure less thantotal—or partial—losses of circulation. Generally, the techniques may beapplied to cure any appropriate types of loss of circulation; however,each situation is unique and should be considered addressed based on itsconditions.

A total loss of circulation may result from faults, such as fractures,in a subterranean formation. Other causes of lost circulation alsoexist. In the example shown in FIG. 2, the drilling fluid, the returnfluid, or both may escape into fractures, such as fracture 22, in asurrounding formation 24, causing the loss of circulation. Dependingupon the size of the fracture and the volume of fluids involved, theescaping fluids may cause a total loss in circulation.

In some implementations, a lost circulation zone may be identified basedon the volume of return fluid removed from a wellbore. For example, thevolume of return fluid, if any, may be measured using one or moredetection mechanisms, and compared to an expected volume of return fluidfor a given amount of drilling fluid pumped into the wellbore. If theamount of return fluid deviates by more than a threshold amount from theexpected amount of return fluid for a given depth in a wellbore, a lostcirculation zone is detected. In some implementations, computer programsmay be used to process information about the volumes of drilling fluidand return fluid, and to make a determination about whether a lostcirculation zone has been encountered. In some implementations, thisdetermination may be made in real-time (such as during drilling) so thatthe situation can be remedied before damage occurs. In someimplementations, the computer programs may be used to alert drillingengineers about a detected lost circulation zone, to begin automaticremedies, or both. In some implementations, a lost circulation zone maybe detected using other methods based on the quantity or quality of thereturn fluid.

In some implementations, lost circulation zones may be remedied, atleast in part, using the cementing operations described in thisspecification. In this regard, drilling cuts through rock formations toform a wellbore that reaches a subterranean reservoir. Cementingoperations may be used to cure a lost circulation zone by pumping cementslurry into the wellbore, and allowing the cement slurry to set, orharden. Following setting, as described, all or some of the drillingfluid may be prevented from escaping into the lost circulation zoneduring drilling.

FIG. 3 shows an example wellbore 28. In the example of FIG. 3, cementslurry is pumped along path 30 to fill a detected lost circulation zone32. As described in this specification, a computer system, such ascomputer system 34, may be programmed to detect the lost circulationzone based on information 36 obtained from sensors inside, or associatedwith, wellbore 28. The computer system may be programmed also todetermine the amounts of high-density fluid and cement slurry to injectinto the wellbore, and the times at which the high-density fluid andcement slurry should be injected. Example wellbore 28 also includes acement retainer 38 in casing 39 to direct the cement slurry. One or morepumps 40 are controllable —computer-controllable or controllable throughother mechanisms—to inject the cement slurry into the wellbore, as shownin FIG. 3. In the example of FIG. 3, the cement slurry 42 in thewellbore abuts formation fluid 44 in fracture 46. This results in a zoneof contamination 48, in which the cement slurry interacts with theformation fluid. In this environment, the cement slurry adjacent tofracture 46 may be unable to set, and to harden. Instead, in some cases,the cement slurry may recede into the lost circulation zone, withoutcuring the lost circulation zone.

Referring to FIG. 4, to address issues described with respect to FIGS. 2and 3, high-density fluid 50 may be pumped into the wellbore prior topumping cement slurry into the wellbore. As shown in FIG. 4, thehigh-density fluid 50 fills at least part of a fault, such as fracture46, in the lost circulation zone 32. The use of the relative term “high”in this context does have any numerical connotation. Rather, thehigh-density fluid has a density that is greater than, or equal to, adensity of the cement slurry, for reasons explained subsequently. Thenumerical value of the density of the fluid may vary based on anyappropriate conditions, such as the density of the cement slurry used,severity of the total losses, the temperature in the wellbore, otherenvironmental conditions, and so forth. In some implementations, thedensity of the high-density fluid is within a range of 65 PCF (poundsper cubic foot) to 130 PCF; however, as noted previously, theappropriate density for that high-density fluid may vary based onvarious conditions, such as cementing conditions. Examples ofhigh-density fluid that may be used include, but are not limited to,oil-based mud and water-based mud. Any appropriate high-density fluidmay be used including, but not limited to, high-density drilling fluid.

In some implementations, the high-density fluid may also have aviscosity that is greater than, or equal to, a viscosity of theformation fluid or of any other fluid that is already located in thewell. As was the case for density, the use of the relative term “high”in this context does have any numerical connotation. Rather, thenumerical value of the viscosity of the high-density fluid may varybased on any appropriate conditions, such as the type and viscosity ofthe drilling fluid or formation fluid in the wellbore, the pressure inthe wellbore, the temperature in the wellbore, the type of rockformations that are encountered during drilling, and so forth.

One or more pumps 40 force the high-density fluid into the wellborealong path 52 and, from there, into the lost circulation zone 32. Insome implementations, the one or more pumps operate at a maximum pumpspeed to force the high-density fluid into the wellbore; however, thehigh-density fluid may be pumped at any appropriate rate. As shown inFIG. 4, the high-density fluid may be pumped into the wellbore along asame path as normal, lower-density drilling fluid. The high-densityfluid may be pumped with sufficient force to displace remaining normal,lower-density drilling fluid in the wellbore. In this regard, inresponse to the high-density fluid, the normal drilling fluid—to theextent that any remains—may be forced into fractures or other faults inthe lost circulation zone.

In some implementations, the high-density fluid may be pumped into thewellbore using a force, and at a volume, that is sufficient to cause thehigh-density fluid to fill the lost circulation zone completely, toenter one or more fractures in the lost circulation zone, and to forceat least some of the formation fluid in each fracture away from a regionthat is adjacent to the wellbore. In this regard, referring, forexample, to FIG. 4, fracture 46 in lost circulation zone 32 may containformation fluid 54. The formation fluid may be native to the lostcirculation zone, and may be fluid such as hydrocarbons or water, or theformation fluid may be, or include, fluid that has been introduced intothe fracture during drilling, such as normal drilling fluid. In anycase, high-density fluid 50 may be pumped, using sufficient volume andforce, to reach the lost circulation zone, and to enter formation 46, asshown. Upon entry into the formation, high-density fluid 50 forcesformation fluid 54 away from the wellbore, and further into thefracture. In some implementations, the high-density fluid may have adensity that is greater than a density of the formation fluid. In someimplementations, the high-density fluid may have a density that is lessthan the density of the formation fluid. As described, the high-densityfluid in the fracture acts as a barrier, of buffer, between the cementslurry and formation fluid.

The amount of high-density fluid to be pumped into the wellbore may varybased on conditions, such as a size of the wellbore, a degree of lostcirculation, a size of the lost circulation zone, and so forth. In someimplementations, the amount of high-density fluid pumped into thewellbore may be in the range of 700 barrels (bbl) to 2000 bbl; however,as noted, this number may vary based on conditions encountered.

In some implementations, the barrier produced by high-density fluid 50is temporary. For example, in some cases, the high-density fluid mayeventually mix with formation fluid 54, dissipate, and escape into thefracture or elsewhere into the formation. Accordingly, the high-densityfluid may be selected or produced based on conditions present in aparticular wellbore. For example, a type of high-density fluid may beselected or produced so that at least some of the high-density fluidremains intact in the fracture to act as a barrier long enough for thecement slurry to set to a threshold hardness. In this regard, somecement slurries takes hours to set, for example, to harden to a pointwhere the cement slurry loses a threshold amount of its plasticity.Accordingly, in cases where the cement slurry takes, for example, aboutfour to five hours to set, a high-density fluid may be selected that isable to maintain a barrier between the formation fluid and the cementslurry for at least four hours. In some implementations, the cementslurry setting time may be more than, or less than, four hours, in whichcase an appropriate high-density fluid may be used that is able tomaintain a barrier between the formation fluid and the cement slurry forat a different period of time, for example, longer or shorter than fourhours.

Following introduction of the high-density fluid into the wellbore,cement slurry 56 is pumped into the wellbore along path 58. The volumeof cement slurry used may be sufficient to cover any appropriate part ofthe wellbore, such as the entire lost circulation zone 32 or a part ofthe lost circulation zone 32. The amount of cement slurry to be pumpedinto the wellbore may be regulated based on conditions such as a size ofthe wellbore, a degree of lost circulation, a size of the lostcirculation zone, and so forth. In some implementations, the amount ofcement slurry to be pumped into the wellbore may be about 300 bbl;however, as noted, this number will vary based on conditionsencountered. In some implementations, the cement slurry is to produce acemented area in a lost circulation zone around an open hole. In someimplementations, the layer of cement produced from the cement slurry ishard enough not to collapse during drilling caused by vibration of thedrill bit and the drill string. Also, the layer of cement produced fromthe cement slurry is sufficient to withstand significant differentialpressure between a subnormal formation pressure and hydrostatic pressureof a mud column during drilling. In some implementations, the layer ofcement produced from the cement slurry is produced to shut an open holein the lost circulation zone.

In some implementations, the cement slurry has a density that is lowerthan the density of the high-density fluid 50 that is present inside theformation in the lost circulation zone, such as a formation 46. As aresult, the high-density fluid, in total or at least in part, isolatesthe cement slurry 56 from the formation fluid 54. For example, thehigh-density 50 fluid prevents, in whole or part, the cement slurry 56from mixing with the formation fluid 54 in the formation 46 for at leasta period of time. Furthermore, because the density of the cement slurryis less than the density of the high-density fluid, any mixing thattakes place between the cement slurry and the high-density fluid wouldtake place over a longer period of time than it takes for the cementslurry to set to a predefined hardness.

In this regard, in FIG. 4, zone 60 is where the formation fluid 54 abutsthe high-density fluid and zone 62 is where the high-density fluid abutsthe cement slurry 56. Due to the density of the high-density fluid,within these zones 60 and 62, there is, at least initially, littlemixing of, or displacement of, adjacent materials. As a result, thecement slurry may remain isolated from the formation fluid for at leastenough time to allow the cement slurry to set. As explained previously,in example implementations like that shown in FIG. 4, any mixing,dilution, or contamination that takes place between the formation fluidand the high-density fluid takes place over a longer period of time thanit takes for the cement slurry to set to the predefined hardness.Accordingly, at least for a period of time, the high-density fluid actsas a barrier to prevent the cement slurry from migrating, andinteracting with, other fluids, allowing the cement slurry enough timeto set. In this regard, in some implementations, there may be smallamounts of formation fluid seeping into the cement slurry, thehigh-density fluid, or both; however, in general, the amounts are smallenough as not to prevent the cement slurry from setting.

In some implementations, pumping the cement slurry into the wellboreimmediately follows pumping the high-density fluid into the wellbore. Insome implementations, pumping the high-density fluid and pumping thecement slurry may be part of a single continuous operation, startingwith pumping the high-density fluid and followed immediately by pumpingthe cement slurry. In some implementations, different pumps or sets ofpumps are used to pump the high-density fluid and the cement slurry intothe wellbore. In some implementations, the same pump or set of pumps isused to pump the high-density fluid and the cement slurry into thewellbore. In some implementations, the pump or pumps arecomputer-controlled based on one or more conditions detected in the wellincluding, for example, the amount of circulation in the well, apressure detected in the well, a temperature detected in the well, otherenvironmental conditions, and so forth.

To perform the cementing operations, in some implementations as shown inFIG. 4, a cement retainer 38 is set at a shoe of a last casing 39 in thewellbore and operates to direct the cement slurry through a drill pipeand open hole into a region of the lost circulation zone. In someimplementations, one or more pumps 40 are configured and controlled topump the cement slurry into the wellbore using a force that issufficient to cause all, or at least some, of the cement slurry to enterthe lost circulation zone, and to enter any, or all, fractures, such asfracture 46. In some implementations, one or more pumps are configuredand controlled to pump the cement slurry into the wellbore using a forcethat is sufficient to cause all, or at least some, of the cement slurryto enter the lost circulation zone, including one or more fractureswithin the lost circulation zone.

In some implementations, for some fractures, such as portion 65 offracture 46, gravity may cause the formation fluid and the high-densityfluid to recede, at least partly, from the wellbore and into thefracture or elsewhere into the formation. This may be the case, forexample, where fractures extend downwardly, towards the Earth's center.In some cases, such as where fractures extend upwardly, away from theEarth's center, such as portion 64, gravity may play less of a role incausing the formation fluid and the high-density fluid to move furtherinto the formation. In any event, as noted, the cement slurry may bepumped into the wellbore using a force that is sufficient to cause atleast some of the cement slurry to enter fractures in the lostcirculation zone. The amount of cement slurry may be regulated—forexample by a computer—so that enough cement slurry is introduced tocover all or part of the lost circulation zone, and to enter one or morefractures. In this regard, in some cases, the cement slurry enters intoa fracture 46 and into contact with high-density fluid contained in thefracture. In some implementations, there is sufficient isolation betweenthe high-density fluid and the cement slurry for the cement slurry inthe fracture to harden and produce cement. Accordingly, in someimplementations, both the sides of the wellbore and the fracture containhardened cement. The cement slurry in the fracture acts as a furtherbarrier to the cement slurry in the wellbore. In some implementations,the cement slurry in the fracture may harden and thereby increase thestability of the formation. If any unset cement slurry in the fractureremains—for example, cement slurry that did not harden—that slurry mayeventually seep into the fracture, along with the high-density fluid.

After the cement slurry hardens to produce the cement bond and seals thelost circulation zone, drilling may continue in the manner describedpreviously. For example, the drilling fluid may be pumped to the drillbit at the bottom of the wellbore. The cementing of the lost circulationzone restores circulation to the well, allowing return fluid, includingmud and other materials, to reach the surface following pumping of thenormal, lower-density drilling fluid.

Referring to FIGS. 5, 6, and 7, a process 66 is shown for producing atleast part of a well using the techniques described previously. Process66 includes identifying (68) a lost circulation zone. Techniques foridentifying the lost circulation zone are described previously. Anexample lost circulation zone 78 in a wellbore 80 is shown in FIGS. 5and 6. Process 66 includes pumping (70) high-density fluid 82 intowellbore 80. Techniques for pumping the high-density fluid are describedpreviously. FIG. 5 shows the high-density fluid 82 in the wellbore. Asshown in FIG. 5, the high-density fluid 82 forces formation fluid 84further into the formation 86. Process 66 includes pumping (72) cementslurry into the wellbore. Techniques for pumping the cement slurry aredescribed previously. As shown in FIG. 6, the cement slurry 88 forcesboth the high-density fluid 82 and the formation fluid 84 further intothe lost circulation zone 86—for example, a fracture—and away from thewellbore. As a result, in this example, the cement slurry 88 occupiesboth the lost circulation zone and a region of the fracture 86 that isadjacent to the wellbore. Process 66 includes waiting (74) for thecement slurry to set to at least a threshold, which may include fullhardening. Drilling (76) of the well may then continue.

Although vertical wellbores are show in the examples presented in thisspecification, the processes described previously may be implemented inwellbores that are, in whole or part, non-vertical. For example, theprocesses may be performed for a fracture that occurs in a deviatedwellbore, a horizontal wellbore, or a partially horizontal wellbore,where horizontal is measured relative to the Earth's surface in someexamples.

All or part of the processes described in this specification and theirvarious modifications (subsequently referred to as “the processes”) maybe controlled at least in part, by one or more computers using one ormore computer programs tangibly embodied in one or more informationcarriers, such as in one or more non-transitory machine-readable storagemedia. A computer program can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a stand-alone program or as a module,part, subroutine, or other unit suitable for use in a computingenvironment. A computer program can be deployed to be executed on onecomputer or on multiple computers at one site or distributed acrossmultiple sites and interconnected by a network.

Actions associated with controlling the processes can be performed byone or more programmable processors executing one or more computerprograms to control all or some of the well formation operationsdescribed previously. All or part of the processes can be controlled byspecial purpose logic circuitry, such as, an FPGA (field programmablegate array) and/or an ASIC (application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only storagearea or a random access storage area or both. Elements of a computerinclude one or more processors for executing instructions and one ormore storage area devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom, or transfer data to, or both, one or more machine-readable storagemedia, such as mass storage devices for storing data, such as magnetic,magneto-optical disks, or optical disks. Non-transitory machine-readablestorage media suitable for embodying computer program instructions anddata include all forms of non-volatile storage area, including by way ofexample, semiconductor storage area devices, such as EPROM (erasableprogrammable read-only memory), EEPROM (electrically erasableprogrammable read-only memory), and flash storage area devices; magneticdisks, such as internal hard disks or removable disks; magneto-opticaldisks; and CD-ROM (compact disc read-only memory) and DVD-ROM (digitalversatile disc read-only memory).

Elements of different implementations described may be combined to formother implementations not specifically set forth previously. Elementsmay be left out of the processes described without adversely affectingtheir operation or the operation of the system in general. Furthermore,various separate elements may be combined into one or more individualelements to perform the functions described in this specification.

Other implementations not specifically described in this specificationare also within the scope of the following claims.

What is claimed is:
 1. A method comprising: pumping fluid into awellbore to fill, at least partly, a region in a lost circulation zone,the fluid having a first density; and after pumping the fluid, pumpingcement slurry into the wellbore, the cement slurry having a seconddensity, the first density being greater than, or equal to, the seconddensity which causes the fluid to prevent, at least partly, the cementslurry from mixing with other fluid in the lost circulation zone for atleast a period of time.
 2. The method of claim 1, where the fluid is afirst fluid; where the method further comprises forming the wellbore bydrilling through a formation using a second fluid, the second fluidhaving a third density, the first density being greater than the thirddensity; and where pumping the first fluid causes the first fluid todisplace at least part of the second fluid in the wellbore.
 3. Themethod of claim 1, where the fluid is a first fluid; where the lostcirculation zone is at least part of a formation containing formationfluid, the other fluid comprising the formation fluid; and where thefirst fluid enters the formation and forces at least part of theformation fluid away from a region adjacent to the wellbore, the firstfluid isolating, at least partly, the formation fluid from the cementslurry.
 4. The method of claim 1, where the lost circulation zoneincludes formation fluid, and where the fluid pumped has a firstviscosity and the formation fluid has a second viscosity, the firstviscosity being greater than the second viscosity.
 5. The method ofclaim 1, where the fluid prevents the cement slurry from mixing with theother fluid in the lost circulation zone for sufficient time to allowthe cement slurry to harden into cement.
 6. The method of claim 1, wherepumping the cement slurry is performed immediately following pumping thefluid.
 7. The method of claim 1, where the period of time is an amountof time that it takes for the cement slurry to set to a thresholdamount.
 8. A method of forming a well through a subterranean formationhaving a fracture containing a formation fluid, the method comprising:pumping a first fluid into a wellbore of the well to force at least someof the formation fluid away from a region of the subterranean formationthat is adjacent to the wellbore, the first fluid having a firstdensity; and pumping cement slurry into the wellbore so that the cementslurry is at least adjacent to the first fluid, the cement slurry havinga second density, the first density being greater than the seconddensity which causes the first fluid to act as a barrier, at leastpartly, between the formation fluid and the cement slurry for at least aperiod of time that is based on a setting time for the cement slurry. 9.The method of claim 8, where the first fluid has a first viscosity andthe formation fluid has a second viscosity, the first viscosity beinggreater than, or equal to, the second viscosity.
 10. The method of claim8, where the period of time is long enough to enable the cement slurryto set to a threshold amount.
 11. The method of claim 8, where the firstfluid is pumped into the wellbore by a pumping mechanism, the pumpingmechanism operating at a maximum pump speed to pump the fluid.
 12. Themethod of claim 8, where pumping the cement slurry is performedimmediately following pumping the first fluid.
 13. The method of claim8, where pumping the first fluid and pumping the cement slurry are partof a single continuous operation.
 14. The method of claim 8, where avolume of the cement slurry that is pumped is based on an amount of thecement slurry needed to fill a lost circulation zone comprising theregion of the subterranean formation.
 15. The method of claim 9, wherethe fracture is part of a lost circulation zone in the wellbore.
 16. Themethod of claim 9, where the formation fluid has a third density, thesecond density being greater than the third density.
 17. A systemcomprising: a detector to identify a lost circulation zone in a wellboreof a well; and one or more pumps that are controllable to performoperations comprising: pumping first fluid into the wellbore to fill atleast part of the lost circulation zone, the first fluid having a firstdensity; and pumping cement slurry into the wellbore, the cement slurryhaving a second density, the first density being greater than the seconddensity causing the first fluid at least partly to prevent the cementslurry from mixing with other fluid in the lost circulation zone for atleast a period of time.
 18. The system of claim 17, further comprising:a casing inside at least part of the wellbore, the wellbore extending atleast partly through a subterranean formation having a feature thatcauses the lost circulation zone, the lost circulation zone containingformation fluid, the other fluid comprising the formation fluid; and acement retainer connected to the casing to direct the cement slurry intothe wellbore; where the one or more pumps are controllable to pump thefirst fluid into the wellbore using a force that is sufficient to causethe first fluid to enter the feature and to displace at least some ofthe formation fluid at a region of the feature that is adjacent to thewellbore; and where the one or more pumps are controllable to pump thecement slurry using a force that is sufficient to cause at least some ofthe cement slurry to enter the feature between the first fluid andwellbore.
 19. The system of claim 17, where the detector comprises acomputing system programmed to receive information representing flow offluid out of the wellbore following flow of fluid into the wellbore. 20.The system of claim 17, where the period of time is an amount of timethat it takes for the cement slurry to set to a threshold amount. 21.The system of claim 17, where the period of time is at least four hours.22. The system of claim 17, where the first fluid has a first viscosity,the other fluid has a second viscosity, and the first viscosity isgreater than the second viscosity.